2. The level measurement instrument of claim 1 wherein the analog
processing circuit comprises a comparator that compares the corrected
receive signal to a select threshold level to develop a data signal
representative of location of a target of interest.

8. The level measurement instrument of claim 6 wherein the direct memory
access controller automatically increments a source address until all
target rejection signal values stored in the memory have been transferred
to the digital to analog converter.

9. A through air level measurement instrument comprising:a sensor
comprising a transducer for transmitting a pulse signal at a target of
interest and receiving reflected echoes of the pulse signal and
developing an analog receive signal representative of the reflected
echoes;an analog processing circuit operatively coupled to the sensor
circuit for receiving the analog receive signal and comprising a summer
subtracting an analog target rejection signal from the analog receive
signal to develop a corrected receive signal; anda programmed digital
processing circuit operatively coupled to the analog processing circuit
and comprising a memory storing target rejection signal value data
representing false target reflections and developing the analog target
rejection signal for transfer to the analog processing circuit.

10. The through air level measurement instrument of claim 9 wherein the
analog processing circuit comprises a comparator that compares the
corrected receive signal to a select threshold level to develop a data
signal representative of location of a target of interest.

11. The through air level measurement instrument of claim 9 wherein the
sensor circuit comprises an ultrasonic transducer.

12. The through air level measurement instrument of claim 9 wherein the
analog processing circuit comprises an envelope detector rectifying the
analog receive signal and a second summer subtracting the analog target
rejection signal from the rectified analog receive signal to develop a
corrected envelope waveform delivered to the programmed digital
processing circuit.

16. The through air level measurement instrument of claim 14 wherein the
direct memory access controller automatically increments a source address
until all target rejection signal values stored in the memory have been
transferred to the digital to analog converter.

17. A through air level measurement instrument comprising:a sensor
comprising a transducer for transmitting a pulse signal at a target of
interest and receiving reflected echoes of the pulse signal and
developing an analog receive signal representative of the reflected
echoes;an analog processing circuit operatively coupled to the sensor
circuit for receiving the analog receive signal and comprising a summer
subtracting an analog target rejection signal from the analog receive
signal to develop a corrected receive signal; anda microcontroller
operatively coupled to the analog processing circuit to develop the
analog target rejection signal and comprising a memory storing target
rejection signal value data representing false target reflections and
developing the analog target rejection signal for transfer to the analog
processing circuit.

18. The through air level measurement instrument of claim 17 wherein the
microcontroller comprises a direct memory access controller transferring
the stored target rejection signal values to a digital to analog
converter to develop the analog target rejection signal.

20. The through air level measurement instrument of claim 18 wherein the
direct memory access controller automatically increments a source address
until all target rejection signal values stored in the memory have been
transferred to the digital to analog converter.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001]There are no related applications.

FIELD OF THE INVENTION

[0002]This invention relates to a process control instrument, and more
particularly, to a measurement instrument with target rejection.

BACKGROUND OF THE INVENTION

[0003]Process control systems require the accurate measurement of process
variables. Typically, a primary element senses the value of a process
variable and a transmitter develops an output having a value that varies
as a function of the process variable. For example, a level transmitter
includes a primary element for sensing level and a circuit for developing
an electrical signal proportional to sensed level.

[0004]An electrical transmitter must be connected to an electrical power
source to operate. One form of such transmitter, known as a four-wire
transmitter, includes two terminals for connection to a power source and
two terminals for carrying the output signal proportional to the process
variable. Where transmitters are remotely located, the requirement for
four conductors can be undesirable due to the significant cost of
cabling. To avoid this problem, instrument manufacturers have developed
devices known as two-wire, or loop powered, transmitters. A two-wire
transmitter includes two terminals connected to a remote power source.
The transmitter loop current, drawn from the power source, is
proportional to the process variable. The typical instrument operates off
of a 24-Volt DC source and varies the signal current in the loop between
4 and 20 milliamps DC. Because of these operating requirements, the
design of the transmitter in terms of power consumption is critical. For
example, when a low level signal of 4 milliamps is transmitted, there is
minimal power available to be consumed by the instrument. Therefore,
circuits must be designed to operate off of such minimal available power.
More recently, designs have been proposed which use wireless technology
for transmitting information on the process variable. Such devices may be
battery powered. Again, the design of the transmitter in terms of power
consumption is critical to avoid premature wearing down of the battery.

[0005]Various industrial distance or level sensing devices operate by
emitting bursts of energy, usually acoustic or electromagnetic, and
measure the time required for reflected echoes to return from the
material surface of interest. The distance is derived from the
propagation speed of the energy burst and the elapsed time of the echo
travel for the echo returning from the target of interest. Recent
instruments of this type use a combination of analog and digital circuits
and include a microcontroller. A microcontroller typically consists of a
microprocessor, sometimes referred to as a central processing unit,
program memory, data memory, and peripheral devices such as analog to
digital and digital to analog converters, memory controllers, serial
communication ports, timers, etc. As noted above, the supply energy may
be very limited. Moreover, in hazardous application environments the
sensor supply energy may be very limited, to preclude the possibility of
igniting flammable substances.

[0006]Signal processing methods that require substantial run time
microprocessor activity to accurately perform echo location tasks
conflict with the requirement that the device consume minimal amounts of
electricity, or result in low measurement update rates. Analog circuit
methods for determining echo time of flight can measure efficiently and
accurately but can be triggered by transient electrical noise or spurious
signals resulting in erroneous measurements.

[0007]Distance measuring devices that operate by emitting bursts of energy
are hindered in many application environments by spurious reflections.
Spurious reflections, also referred to as false targets, are usually
caused by extraneous objects that reflect the emitted energy. They may
also result from unintended return paths. A measurement instrument must
be capable of rejecting spurious reflections or its suitable applications
will be limited.

[0008]Signal processing devices that require substantial run time
involvement of a microprocessor have been used to reject spurious
reflections by, for example, continuously digitizing echo signals and
processing the acquired data mathematically. However, applications that
allow for minimal amount of electricity limit available digital
processing power.

[0009]The present invention is directed to improvements in measurement
instruments.

SUMMARY OF THE INVENTION

[0010]In accordance with the invention, there is provided a process
measurement instrument with target rejection.

[0011]There is disclosed in accordance with one aspect of the invention a
process measurement instrument with target rejection comprising a sensor
circuit. The sensor circuit comprises a drive circuit for transmitting a
pulse signal at a target of interest and a receive circuit receiving
reflected echoes of the pulse signal and developing an analog receive
signal representative of the reflected echoes. An analog processing
circuit receives the analog receive signal and comprises a summer
subtracting an analog target rejection signal from the analog receive
signal to develop a corrected receive signal. A programmed digital
processing circuit is operatively coupled to the analog processing
circuit and comprises a memory storing target rejection signal value data
representing false target reflections and developing the analog target
rejection signal for transfer to the analog processing circuit.

[0012]It is a feature of the invention that the analog processing circuit
comprises a comparator that compares the corrected receive signal to a
select threshold level to develop a data signal representative of
location of a target of interest.

[0013]It is another feature of the invention that the sensor circuit
comprises an ultrasonic transducer.

[0014]It is a further feature of the invention that the analog processing
circuit comprises an envelope detector rectifying the analog receive
signal and a second summer subtracting the analog target rejection signal
from the rectified analog receive signal to develop a corrected envelope
waveform delivered to the programmed digital processing circuit. The
digital processing circuit may periodically update the stored target
rejection signal value data using the corrected envelope waveform.

[0015]It is a further feature of the invention that the digital processing
circuit comprises a direct memory access controller transferring the
stored target rejection signal values to a digital to analog converter to
develop the analog target rejection signal. Timing of the analog target
rejection signal may be controllably varied responsive to signal
propagation speed changes. The direct memory access controller may
automatically increment a source address until all target rejection
signal values stored in the memory have been transferred to the digital
to analog converter.

[0016]There is disclosed in accordance with another aspect of the
invention a through air level measurement instrument comprising a sensor
comprising a transducer for transmitting a pulse signal at a target of
interest and receiving reflected echoes of the pulse signal and
developing an analog receive signal representative of the reflected
echoes. An analog processing circuit is operatively coupled to the sensor
circuit for receiving the analog receive signal and comprising a summer
subtracting an analog target rejection signal from the analog receive
signal to develop a corrected receive signal. A programmed digital
processing circuit is operatively coupled to the analog processing
circuit and comprises a memory storing target rejection signal value data
representing false target reflections and developing the analog target
rejection signal for transfer to the analog processing circuit.

[0017]There is disclosed in accordance with another aspect of the
invention a through air level measurement instrument comprising a sensor
comprising a transducer for transmitting a pulse signal at a target of
interest and receiving reflected echoes of the pulse signal and
developing an analog receive signal representative of the reflected
echoes. An analog processing circuit receives the analog receive signal
and comprises a summer subtracting an analog target rejection signal from
the analog receive signal to develop a corrected receive signal. A
microcontroller is operatively coupled to the analog processing circuit
to develop the analog target rejection signal and comprises a memory
storing target rejection signal value data representing false target
reflections and developing the analog target rejection signal for
transfer to the analog processing circuit.

[0018]Further features and advantages of the invention will be readily
apparent from the specification and from the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a side elevation view of a measurement instrument in
accordance with the invention mounted in a process vessel;

[0020]FIG. 2 is a partial block diagram of the instrument of FIG. 1;

[0021]FIG. 3 is a block diagram of a sensor assembly of the instrument of
FIG. 1;

[0022]FIG. 4 is a set of curves illustrating a receive signal with and
without target rejection in accordance with the invention;

[0023]FIG. 5 is a diagram illustrating measurement system control flow
implemented in the microcontroller of FIG. 2; and

[0024]FIG. 6 is a set of curves illustrating analog and digital signals
for the process instrument of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0025]Referring to FIG. 1, a process instrument 10 according to the
invention is illustrated. The process control instrument 10 uses bursts
of energy for measuring a level based on time required for reflected
echoes to return from the material surface of interest. Particularly, the
instrument 10 uses through air ultrasound for sensing level.

[0026]While the embodiments described herein relate to an ultrasonic level
sensing apparatus, various aspects of the invention may be used with
other types of process control instruments for measuring various process
parameters, such as a distance or range finder, as will be apparent to
those skilled in the art. Moreover, the instrument may use other forms of
energy where the instrument measures time required for reflected echoes
to return from the material surface of interest. Distance is derived from
the propagation speed of the energy burst and the elapsed time of the
echo travel for the echo returning from the target of interest. This
distance can represent location of the material surface of interest, and
thus can represent level by knowing the distance from the instrument
relative to distance to the bottom of a vessel, or measuring flow when
used in connection with a flume, as is known.

[0027]The instrument 10 includes a control housing 12 and a transducer 14
connected by a threaded fitting 18 for connection to a process vessel V.
In accordance with the invention, a signal S in the form of bursts of
energy is emitted from the transducer 14 and subsequently reflected from
a surface of the material M. The distance is calculated by the equation

D=(velocity of S propagation)*transit time (round trip)/2.

Level is then calculated by applying a tank height value, where L=H-D.

[0028]In accordance with the invention, the instrument 10 utilizes
improved measurement system design embodying false target rejection by
combining digital signal processing and analog hardware signal detection.
The received analog signal generated from the emitted energy reflections,
in the form of echoes, is used for the primary time of flight
measurement. Derived envelope waveform data is acquired by the
instrument's microprocessor and used to determine false target rejection
data. The false targets may comprise appurtenances in the vessel, such as
ladders, mixers, etc. This system accomplishes accurate and robust
measurement very efficiently.

[0029]In accordance with the invention, portions of an echo signal
waveform that correspond to false target reflections can be captured and
digitized by a microcontroller using an analog to digital converter (ADC)
and stored in non-volatile memory. Once captured, the false target signal
data can be converted back to an analog signal using a digital to analog
converter (DAC) and then inverted and combined with dynamic echo signals
using analog mixing hardware to effectively eliminate the spurious
reflections. The output of the target rejection signal over time can be
achieved by employing a direct memory access (DMA) controller, as a
microcontroller peripheral, in combination with a timer to transfer data
to a DAC register. If necessary, the output timing of the target
rejection signal data can be adjusted to compensate for changes in
propagation speed. Other than initial set up of various microcontroller
registers and one time processing of the signal data, this method does
not require involvement of the microprocessor or CPU, which can therefore
operate more effectively in multi-tasking environments and at lower clock
rate, thereby reducing power consumptions.

[0030]Referring to FIG. 2, a block diagram of a measurement system 20
contained in the instrument control housing 12 is illustrated. This
measurement system 20 is operatively connected to a sensor circuit 22,
see FIG. 3, contained in the transducer 14.

[0031]The sensor circuit 22 includes a piezo-electric crystal 24 connected
to a drive circuit 26 and to a filter circuit 28. The filter circuit 28
is connected to a differential preamplifier 30. The drive circuit 26
receives a drive signal from a microcontroller 32, see FIG. 2, in the
form of a 60 KHz pulse signal to drive the crystal 24. Likewise, the
crystal 24 receives reflected echo pulses which are passed through the
filter 28 to the preamplifier 30. The preamplifier 30 receives a
reference from a reference circuit 34 and develops a 60 KHz signal
comprising an analog receive signal representative of the reflected
echoes, as is well known. The analog receive signal is supplied on a line
36 to a receive signal block 38 of the measurement system 20, see FIG. 2.

[0032]As described, the sensor circuit 22 comprises the drive circuit 26
transmitting a pulse signal via the crystal 24 at a target of interest
and a receive circuit in the form of the crystal 24, filter 28 and
preamplifier 30 receive reflected echoes of the pulse signal on the line
36 representative of the reflected echoes and develop the analog signal
on the line 36. In the illustrated embodiment of the invention, the
sensor circuit 22 includes a single crystal 24. As is known, the sensor
circuit 22 could use separate transmit and receive crystals.

[0033]Referring particularly to FIG. 2, the measurement system 20
comprises an analog processing circuit 40 and a programmed digital
processing circuit in the form of the microcontroller 32. The analog
processing circuit 40 is operatively coupled to the sensor circuit 22,
see FIG. 3, by the receive signal block 38 receiving the analog receive
signal on the line 36. The analog receive signal is illustrated by the
exemplary curve 38'. An envelope detector 42 receives the analog receive
signal from the receive signal block 38. The envelope detector 42
comprises a rectifier with low pass smoothing and develops an envelope
waveform at a block 44. A representation of the envelope waveform,
corresponding to the receive signal curve 38', is illustrated by a curve
44'.

[0034]The analog receive signal from the block 38 is also supplied to a
mixer in the form of a summer 46. The summer 46 also receives a target
rejection signal from a block 48. The target rejection signal is
illustrated by a curve 48'. As described below, the target rejection
signal 48' corresponds generally to the receive envelope 44' but having
only negative pulses corresponding to false target reflections. The
summer 46 adds the receive signal 38' and the target rejection signal 48'
to develop a corrected receive signal at a block 50. The corrected
receive signal is illustrated by the curve 50'. In the illustrated
example, the receive signal 38' includes a pulse A representing the
transmitted pulse, and echoes B, C and D. Echoes B and C comprise false
target reflections. Pulse D represents the reflected echo for the target
of interest. The target rejection signal includes corresponding negative
pulses B' and C' corresponding to the false target reflections. Thus,
when the receive signal 38' and the target rejection signal 48' are
summed by the adder 46, the false target echoes B'' and C'', see curve
50', are of approximately double the magnitude, but on the negative side
only. The corrected receive signal from the block 50 is supplied as one
input to a comparator 52. The other input of the comparator 52 is
generated by a CPU 54 of the microcontroller 32. Particularly, the CPU 54
generates a select threshold level signal. The comparator circuit
compares the corrected receive signal 50' to the selected threshold level
to develop a level data signal on a line 56 also supplied to the CPU 54.
The level data signal on the line 56 is a discrete signal which used by
the microcontroller 32 to start and stop the timer 64 to indicate the
round trip transit time, discussed above. Particularly, the time of
flight measurement is made by the timer 64 which counts clock ticks from
a start pulse until the return echo is detected by the analog comparator
circuit 52 which latches the level data signal 56 high when the echo
signal has exceeded the threshold, stopping the timer 64. Because the
false target echoes B'' and C'' are negative, they will be below the
threshold. Instead, the comparator 52 is latched only by the transmitted
pulse A and the target echo D. Thus, the corrected receive signal 50''
effectively eliminates the false target echoes from the received signal
by insuring they are below the threshold.

[0035]FIG. 4 illustrates the measurement system 20 under typical operating
conditions, with no target rejection at the top, and with target
rejection enabled at the bottom. In the absence of target rejection, the
full echoes B, C and D are illustrated. The corrected receive signal 50'
illustrates the substantially reduced false target reflections B'' and
C'' which will be well below the threshold for determining time of
flight. As noted, with no target rejection, the second false target C
would be detected by the comparator 52. Thus, the false target rejection
in accordance with the invention ignores the false targets and instead
operates only on the target of interest represented by the echo D.

[0036]The target rejection signal 48' from the block 48 is also supplied
to a second summer 58. The receive envelope 44' is a second input to the
second summer 58. Thus, the second summer 58 adds the receive envelope,
see the curve 44', and the target rejection signal, see the curve 48', to
develop a corrected envelope waveform at a block 60. The corrected
envelope waveform is illustrated by the curve 60'. The corrected envelope
waveform 60' differs from the receive envelope waveform 44' as by
eliminating the false target reflections. The corrected envelope waveform
is input to an ADC of the microcontroller 32.

[0038]Generation of the target rejection signal at the block 48 is
achieved using the DAC 68 which produces a series of voltages
representing a waveform. The DAC 68 is driven by the DMA controller 66 to
transfer voltage values corresponding to a previously acquired waveform,
in the form of a target rejection profile, from the memory 62 to the DAC
68. The timing of the waveform voltage value as transferred by the DMA
controller 66 is determined by the hardware timer 64 which is set to
trigger the DMA controller 66 periodically in conjunction with the drive
signal to the sensor 22, discussed above.

[0039]The signal output timing is nominally equivalent to the sample rate
of the previously acquired target rejection profile. However, the output
timing is expanded or contracted in the time dimension when the signal
propagation speed changes. The propagation speed of an energy burst
through a vapor space or other medium can vary, for example, due to
changes to the medium density or temperature, which can be measured.
Expansion or contraction of the target rejection signal is achieved by
changing the period of the hardware timer that drives the output
proportionally to the ratio of the propagation speed in effect when the
target rejection profile was captured and the extant propagation speed,
in accordance with the following equation:

[0040]FIG. 5 illustrates the control flow implemented in the
microcontroller 32 of FIG. 2 for the target rejection task during a scan.
The flow illustrates the target rejection task in the left column and
other tasks implemented in the microcontroller 32 in the right column.

[0042]The target rejection task begins with a node 100 to set up the DMA
controller. This comprises setting a DMA channel for forward source
address/fixed destination address single transfer operation. The source
address is set to the memory location of the target rejection profile
data and the destination address to the DAC register address. The DMA
transfer trigger is set as hardware timer underflow. The DMA transfer
count is set to the number of target rejection profile data points. A
node 102 then sets up the timer. This comprises setting the input clock
source and timer mode as self re-loading. The timer value is set to the
desired period. This can be adjusted for propagation speed changes, if
necessary. The DMA transfer is then started at a node 104. This comprises
enabling the DAC 68 and DMA channel. The timer 64 is then started and is
synchronized to the start of the measurement scan. As is apparent, the
microcontroller 32 may perform other tasks during the transfer which
operates independently of the CPU 54. Transfer is completed at a node
106.

[0043]Once the timer 64 is started, the DMA transfer proceeds to
completion, automatically incrementing the source address until all
target rejection signal values have been transferred to the DAC 68. The
DMA channel, hardware timer and DAC are then disabled until the next
measurement scan.

[0044]FIG. 6 depicts the acquisition of the target rejection profile data
required for the target rejection feature. The analog echo envelope
waveform signal at the block 44 is sampled during measurement scans in
order to make echo waveform data available for false target rejection,
and other system purposes. This is done using the waveform data acquired
by the ADC 70. The microcontroller 32 digitizes the envelope waveform
signal using the ADC 70. This is illustrated at the top portion of FIG.
6. The data is precisely timed and could also use a DMA controller for
efficiency. The data is accumulated as a running average of multiple
acquired envelope waveform traces to provide a stable waveform
representation as single waveform scans are not always reliable in real
world applications due to environmental disturbances to the liquid
surface and/or vapor space properties.

[0045]A portion of the acquired running average echo waveform data is
designated for use as a target rejection profile when the target
rejection routine is invoked by the device operator. The portion of the
envelope waveform designated for the targeted rejection profile ideally
corresponds to the majority of the measurement region of the application
in order to capture all possible false target reflections. However, this
requires the vessel to be empty. For partially filled situations, the
exposed portion of the measurement region, the area between the
transducer and the liquid surface, can be used. The portion of the
average envelope data selected for the target rejection profile is
smoothed and amplified, due to gain differences in the amplifier
circuits, to create the target rejection profile. This target rejection
profile data is stored in system memory 62 and is output to the analog
circuit board where it is combined with the receive signal, as described
above. The system can periodically update, automatically or based on user
intervention, the target rejection signal value data used on measured
feedback. In doing this, the target rejection signal would be effectively
disabled to eliminate target rejection to determine the false target
reflections, as shown in FIG. 6.

[0046]Thus, in accordance with the invention, the measurement cycle is
implemented in the analog processing circuit 40. The microcontroller uses
the DMA controller 66 to more efficiently transfer out the target
rejection signal minimizing the use of the CPU 54. The computationally
intense updating of target rejection data is performed infrequently to
thus improve overall measurement system robustness.

[0047]Thus, in accordance with the invention, there is provided a
measurement system with target rejection combining digital signal
processing and analog hardware signal detection.